The invention relates to a method for producing multilayered optical systems by a one-step baking method (xe2x80x9cstack bakingxe2x80x9d).
Multilayered systems having an optical effect can be produced on glass by the so-called sol-gel process; see, for example, Dislich et al. DE 1941191. The principle of this production method is that the corresponding glass substrate is coated with a sol by an immersion process, dried at elevated temperatures and baked in order to achieve densification. After pre-drying at temperatures of  greater than 100xc2x0 C., it is also possible to apply a further layer by immersion and to bake the two layers in one step. The pre-drying at elevated temperatures is necessary in order to provide the first layer with adequate chemical stability, since it is otherwise partially or fully dissolved by the new coating sol. The layer thickness which can be achieved with a plurality of layers, without prior densification of each individual layer by sintering, is about 0.5 xcexcm, since otherwise cracking occurs. The cracking occurs as a consequence of the already strong three-dimensional crosslinking of the porous layer systems, since the shrinkage which occurs on heating can no longer be dissipated by stress relaxation. In addition, the method is also complex since heat treatment is necessary after each layer application and in the case of a plurality of layers, baking at temperatures of 400-500xc2x0 C. becomes necessary. The production of multilayered systems, as necessary in specific optical applications (broad-band antireflection coating, diathermic mirrors, etc.), becomes extremely labour-intensive and expensive.
Although it has been shown in WO 93/24424 that the incorporation of relaxation mechanisms enables thick layers to be produced as well, these cannot, however, be used to achieve an optical effect since they do not satisfy the condition of quarter-wave (xcex/4) layers.
The object of the invention was to produce multilayered optical systems as far as possible without complex intermediate heating steps in such a way that the optical effect later desired occurs.
This object is achieved in accordance with the invention by a method for producing multilayered optical systems which comprises the following steps:
a) application of a flowable composition comprising nanoscale inorganic solid particles containing polymerizable and/or polycondensable organic surface groups to a glass substrate;
b) polymerization and/or polycondensation of the surface groups of the solid particles with formation of an organically crosslinked layer;
c) application to the organically crosslinked layer of a further composition in accordance with a) which produces a different refractive index to the previous composition;
d) polymerization and/or polycondensation of the surface groups of the solid particles with formation of a further organically crosslinked layer;
e) if desired, repetition of steps c) and d) one or more times with formation of further organically crosslinked layers on the organically crosslinked layers already present and/or on other surfaces of the substrate; and
f) one-step thermal densification of the multilayer system and removal of the organic constituents present by baking.